Comcast Wrongfully Denies Interfering with BitTorrent

Last week we reported that Comcast is making it impossible for its customers to seed files on BitTorrent. Not surprisingly, Comcast's PR department does all it can to deny there allegations, but we - and with us some of the leading BitTorrent developers - know better.

So who’s right here? The hundreds of people that seem to have the same seeding problem, or the Comcast spokesman Charlie Douglas, who denies that Comcast is monkeying with BitTorrent bandwidth. Apparently Comcast want people to believe the latter, even though all evidence points in the other direction. Decide for yourself.

Moxie, a Comcast customer who replied to a post on Silicon Alley, points out that when you log your network activity with an application like Wireshark, you’ll notice that Comcast servers start sending reset messages as soon as a download is finished, exactly as we described it.

With Wireshark in the background run your BitTorrent application. Wait until completed and watch Wireshark, notice when it finishes seeding Comcast servers send out a reset command every second to your computer noted by the highlighted red line in Wireshark. It is 8:30 pm Monday pst and Comcast is still resting my BitTorrent connections. Maybe the PR guy didn’t get the email from the VP of Networking.

It might be that not every Comcast customer is equally affected, but a significant percentage is. Not only the 10+ users we talked to before we first reported this issue, but also hundreds of additionalcommenters here on TorrentFreak, and elsewhere. Some users even captured the throttling in progress on video (download), and anyone has to agree that this does look very suspicious.

More evidence comes from Robb Topolski, a networking and protocol expert with more than 25 years of experience, who first wrote about this issue on DSLReports. He told TorrentFreak: “We have had two Comcast techs confirm Sandvine in use, but neither confirmed or denied its connection with the RST interference. For me, seeding is possible. I can reach my upload speed limit, but there sure is a lot of interference. Since your article came out, I too have received many reports of seeding being impossible. I’m not sure if it’s regional, or what!”

For the networking savvy people among us, here’s an example of real RST interference on an unencrypted BitTorrent connection. In this case, it happens right after the bitfields are exchanged

Nevertheless Comcast spokesman Charlie Douglas said in a response to Light Reading: “We’re not blocking access to any application, and we don’t throttle any traffic”. He might be right here semantically speaking, they are not throttling anything, they just kill all outgoing connections when a clients starts to seed a file. But the fact is that Comcast is making it impossible for (at least some) customers to share files with non-Comcast users over BitTorrent.

Luckily there is a fix for this problem, and we know that at least two BitTorrent client developers are including this fix in their next update.